1. Field of the Invention
This invention relates to centrifugal pumps.
2. Description of the Prior Art
Many designs of centrifugal pumps have heretofore been proposed.
Centrifugal pumps, because of their simplicity, low cost and ability to operate under a wide variety of conditions are extensively used as fluid handling devices. Such pumps are adapted to operate under a variety of heads up to several thousand feet, will handle liquids even at elevated temperatures, and operate at speeds that are standard for electric motors. Such pumps also can be throttled at the discharge without building up excessive pressure or overloading the driving unit.
The type of centrifugal pump most commonly used by industry because of its variability is the volute type with a variant known as the diffuser type. The volute type pump has a progressively expanding spiral casing into which the impeller discharges, the casing being proportioned to produce substantially equal velocity flow around its circumference and to gradually reduce the velocity of the liquid as it flows from the impeller to the discharge connection thereby changing velocity head to pressure head.
In the diffuser type of pump, the runner is surrounded by gradually expanding passages in the casing, formed by stationary guide vanes, for changing the direction of flow and converting velocity head to pressure head before the liquid enters the volute. A more complete conversion of velocity head to pressure head is effected, with increase of efficiency but at added cost and complication of the structure. Accordingly, diffuser type pumps are not considered justified except in large high pressure pumps.
In the volute type pumps commonly employed the efficiency is not high even when the volute and impeller are perfectly mated, at the highest efficient design point.
It is a common practice within this segment of the pump industry for the manufacturer, for economic reasons, to build a minimum number of volutes and vary the performance of the pump by variations in the width and diameter of the impeller. This approach results in lower operating efficiencies and renders ineffective the concept of producing equal velocity flow around the circumference of the volute. Resort has been had to the use of diffusers as mentioned above to overcome the large loss in efficiency. Problems of variability arise since the impellers, at least in diameter, must be reasonably matched to the inside dimensions of the volute vanes.
In the volute type pump, also, provisions have beem made for using wider impellers but this results in large areas for fluid rotation with turbulence and high friction losses.
The almost universal practice in such pumps in the range of 1 to 10 horsepower, and higher, has been to employ cast volutes, usually with integral cast inlet and outlet connections.
It has also heretofore been proposed to employ, in place of volutes, casings of circular transverse cross section but these have had very low efficiencies and have only been used for small pumps.
The customary pump design employing a volute with increasing area from the cut water to the discharge for reduction of velocity and conversion to pressure leaves much to be desired in practical applications and from the basic design viewpoint. The gradually increasing area in the volute is not required from the standpoint of flow since an examination of the performance curves for such pumps makes it clear that as the head increases the flow decreases so that such area increase is not required.
From the standpoint of efficiency of hydraulic flow and conversion of energy the utilization of a casing which does not increase in area is advantageous. The use of a concentric or circular casing has other benefits from the standpoint of manufacturing design, cost of construction, and utilization of materials of construction that are either expensive or difficult to obtain in forms that would be used in castings. Additional advantages follow from the use of concentric and symmetrical casings in permitting the use of manufacturing procedures employing standard machine tools, and from the strength advantage in having pressure stresses distributed equally on the casing.
The present invention departs from prior structures in that, in order to obtain greater efficiencies in the use of a circular casing, an inserted cut water plate or tongue is employed readily shaped as desired and required which not only has the periphery of the impeller running close to one portion but has portions disposed along the sides of the impeller substantially to the wearing ring with sufficient clearances to prevent interference in operation of the impeller. The cut water plate or tongue can be readily matched in manufacture to the width and diameter of the impeller thereby reducing the number of casing sizes required for a wide range of impeller sizes and providing greater versatility for casing utilization. The use of such a cut water plate or tongue in eccentric or volute casings will also improve the efficiencies of pumps of that type.